Antimicrobial Activity of Thymol against Pathogenic Gram-negative Bacteria of Fishes

This work investigated the antimicrobial effects of thymol in vitro against seven species of Gram-negative fish pathogenic bacteria namely, Aeromonas salmonicida subsp. masoucida, A. salmonicida subsp. salmonicida, A. hydrophila, Edwardsiella tarda, Vibrio vulnificus, V parahaemolyticus and V anguillarum using disk diffusion, minimum inhibitory concentration (MIC) and minimum bacteriocidal concentration (MBC) tests. In the disk diffusion test, thymol inhibited growth of all bacteria including those known to be resistant to amoxicillin or lincomycin or both. MIC or MBC of thymol against all bacteria were in the range of 0.01 mg/ml to 0.32 mg/ml. The most sensitive was A. salmonicida subsp. salmonicida (0.01 mg/ml for MIC and 0.02 mg/ml for MBC), followed by A. salmonicida subsp. masoucida (0.04 mg/ml for MIC and 0.08 mg/ml for MBC). Based on the present results, thymol has the potential of controlling bacterial pathogens in the aquaculture industry.